How to create a curved surface for Savonius VAWT?
To me they just look like three half cylinders, rotated by 120°.
Add a cylinder, select the top and bottom face and half of its shell and delete them.




Then select the remaining faces and move them so that the last straight edge is located on the center of the scene. Make sure the 3D cursos is located at the center as well. If it is not, hit [Shift+S] and select "Cursor to Center". Make the cursor your current transformation center by pressing [.]


Duplicate the vertices by pressing [Shift+D] and immediatly press [Esc] to cancel translation. Rotate the selected vertices by pressing [R] and moving your mouse or by typing the desired angle of 120 degrees on your keyboard. If you chose to use the mouse, you can hold [Ctrl] to make Blender increment the angle in five degree steps. This makes aiming for 120° easier. Hit enter to apply the rotation. Duplicate the selected vertices again and rotate them in the same manner.
The result should look something like this:


If you want to give it some thickness, go to the modifiers tab and select the solidify modifier: